Isomorphism and endocrypty: Novel approaches in studies of behavior of microelements in mineral systems

The known notions of limiting concentration of impurities and isomorphic ca pacity of minerals have been applied to defective crystals using the concep t of endocrypty, reinterpreted as a mode of incorporation of impurity atoms into the real structure of host minerals. It is shown that experimental st udies of mineral systems containing microelements (ME) should be based on a nalysis of factors facilitating structural incorporation of ME into mineral s. Limiting concentration of ME is correlatable with maximum density of def ects in a crystal which causes the strongest influence on incorporation of the impurity and can be estimated on this basis for different specific cond itions; extrapolation to zero defect density shows isomorphic capacity of a mineral with respect to a given ME. In particular cases when a microelemen t cannot form a separate phase limiting its solubility in the host mineral, it is proposed to use special components which are additional elements or complexes facilitating saturation of the fluid phase and the respective cry stallizing mineral by combining with ME to form intermediate phases. This a pproach has a difficulty in discriminating modes of occurrence of ME in a s aturated phase. This difficulty can be overcome if considering undersaturat ing ME concentrations in the study mineral when it is in phase equilibrium with a "reference" mineral allowing high solubility of these microelements. In general case, besides the curve of the true phase equilibrium correspon ding strictly to isomorphism of ME, there are a number of additional appare nt phase equilibrium curves depending on the types of crystal defects and t heir role in endocrypty. Correspondingly, the constant of impurity distribu tion can deviate from the true isomorphic cases. Thus, in the case of endoc rypty, the limiting concentration of incorporated ME is not constant at spe cific P and T values but depends on the conditions under which the real cry stal was forming and on the character of its defects. It is suggested to ap ply the earlier proposed concept of forced equilibria to analyze different modes of incorporation of ME attributed to structural defects of minerals.